make runs target even though file already exists

I have a Makefile like this

bin:
	mkdir -p bin

bin/kustomize: bin
	curl -fsSL "https://raw.githubusercontent.com/kubernetes-sigs/kustomize/master/hack/install_kustomize.sh" | bash -s bin

When I run make bin/kustomize it tries to download it, every time, even though it is already there. I would expect that make doesn't want to run that target.

When I remove the dependency to bin, it works as I expect.

Your bin/ is a directory, not a plain file.

The GNU make dependency in your rule

bin/kustomize: bin

says that file bin/kustomize should be reconstructed every time the modification time of bin/ is changing.

But overwriting a file entry (technically associating a name to an an inode) inside a directory is changing the directory modification time.

As commented by HolyBlackCat you want an order-only prerequisite

Should you want the building rules to fire when a content is changed (and not just using modification time), consider using other builder software, maybe omake

An example of a more complex GNU makefile is inside the RefPerSys open source inference engine.